Transonic buckets for axial flow turbines

ABSTRACT

TURBINE BUCKET PROFILES FOR THE TRANSONIC FLUID FLOW REGION AT THE OUTER PORTIONS OF AXIAL FLOW STEAM TURBINE BUCKETS, THE PROFILES BEING CHARACTERIZED BY SUBSTANTIALLY FLAT PRESSURE AND SUCTION SIDES DIVERGING TOWARD THE TRAILING EDGE OVER THE MID-PORTION OF THE PROFILE, THE BUCKETS OVERLAPPING ONE ANOTHER AND SHAPED TO PROVIDE A TRANSONIC AND SUPERSONIC FLOW REGION BETWEEN BUCKETS.

1971 J. E. FOWLER E 1 3,565,543

TRANSONIC BUCKETS FOR AXIAL FLOW TURBINES Filed Jan. 24, 1969 lNVENTORSJACKSON E. FOWLER,

JOSEF HERZOG, BY 40 W THEIR ATTORNEY.

United States Patent O 3,565,548 TRANSONIC BUCKETS FOR AXIAL FLOWTURBINES Jackson E. Fowler, Schenectady, and Josef Herzog, Scotia,

N.Y., assignors to General Electric Company, a corporation of New YorkFiled Jan. 24, 1969, Ser. No. 793,831 Int. Cl. F01d 5/14 US. Cl. 416-2235 Claims ABSTRACT OF THE DISCLOSURE Turbine bucket profiles for thetransonic fluid flow region at the outer portions of axial flow steamturbine buckets, the profiles being characterized by substantially flatpressure and suction sides diverging toward the trailing edge over themid-portion of the profile, the buckets overlapping one another andshaped to provide a transonic and supersonic flow region betweenbuckets.

BACKGROUND OF THE INVENTION This invention relates to axial flow steamturbine bucket design for high speed, long buckets wherein the fluidflow conditions at the radially outer portions, especially of the laststage buckets, exceeds the speed of sound. Such buckets are known astransonic buckets because the steam enters at a speed less than that ofsound and emerges at supersonic speeds.

Transition from subsonic to supersonic conditions without the loss ofetficiency represents a diificult aerodynamic design problem. That whichwould give a theoretical optimum fiow condition is often impossible toachieve in a practical blade due to thinness of the blade sections nearthe outer tips of the buckets. Efforts to achieve good transonicprofiles are known in the prior art, wherein a conventional subsonicbucket profile (concave on the pressure side and convex on the suctionside) has been reversed so that there is a slightly convex curve on thepressure side and slightly concave curve on the suction side. Theseprofiles and a history of the prior art are exemplified in US. Pat.3,333,817 issued to F. Rhomberg on Aug. 1, 1967. Such prior art bladeprofiles terminate in a relatively thick trailing edge which is disposedopposite a blunt leading edge of the adjacent blade.

Accordingly, one object of the present invention is to provide animproved transonic bucket profile which gives improved efficiency in thetransition from subsonic to supersonic flow between buckets.

Another object of the invention is to provide an improved transonicbucket profile which is of more uniform thickness throughout so as toadd strength without sacrifice in aerodynamic design.

DRAWING The single figure of the drawing is a developed view of twoadjacent blade cross sections taken near the outer tips of the bucketsand looking radially inward.

SUMMARY OF THE INVENTION Briefly stated, the invention is practiced byarranging adjacent blades so that the blade sections near the outerperiphery overlap about A of the total chord length and shaping theblade profiles a convergent-divergent nozzle over the overlappedone-quarter. Each blade profile is characterized by two locations ofincreased blade thickness, one near the nose and one near the tailsection. Over the midsection of the blade profile, the pressure side andsuction side are substantially fiat and diverge from one another towardthe tail section.

3,565,548 Patented Feb. 23, 1971 DESCRIPTION OF THE PREFERRED EMBODIMENTReferring now to the drawing, there is shown a cross section of twoadjacent buckets, designated 1 and 2, taken near the bucket outer tips.It will be understood by those skilled in the art that the configurationand orientation near the radially inner ends of these same buckets willbe quite different in appearance and that there is a gradual transitiontoward the profiles indicated in the drawing. Each bucket profile, suchas 2, is divided for the purpose of definition herein into a nosesection 3 extending about A of the chord length, a mid-section 4extending about /2 of the chord length, and a tail section 5 extendingthe remaining of the chord length. The nose section 3 of one bladeoverlaps and is roughly coextensive with the tail section 5 of anadjacent bucket.

In the drawing, the blades travel in the direction indicated by thearrow. The upper side 6 is the pressure side of the blades while thelower side 7 is the suction side.

In the nose section 3, the point of greatest blade thick ness isindicated at a and is located back from the leading edge 8 by distanceI. The contour of the blade profile on both pressure and suction sideover the distance I is quasielliptical and has an l/d ratio satisfyingthe condition:

From the point of greatest thickness d, the blade narrows toward a neck11 at the start of the mid-section 4 of the blade. The suction side inthis narrowing cross section is gently concave as indicated at 12.

Over the mid-section 4 of the blade, the pressure side and suction sideare substantially fiat. It is to be particularly noted also that theflat pressure and suction sides diverge from the relatively narrow neck11 at the beginning of mid-section 4 to a relatively thick throatportion indicated at its thickest point as t. Throat portion t islocated approximately /4 of the chord length along the blade and is sodesignated because it lies opposite the thickest nose section d to formtherewith the throat of a. convergent-divergent nozzle between blades 1,2.

In some cases, the nose thickness d will be slightly greater than t andin other cases the throat thickness 1 will be slightly greater than a.In either event, however, the two are approximately the same, relativeto other parts of the blade.

The tail section 5 of the blade profile comprises a relatively flatsuction side which is a continuation of the aforementioned flat suctionside over the mid-section of the blade. On the pressure side, the bladecurvature is reversed after passing through the throat portion t andcaused to curve gently toward the trailing edge 9 to provide a slightlyconcave surface on the pressure side. The radius of curvature over thisportion, designated 10, should be at least 20 times that of the radiusof curvature at the throat section I.

OPERATION The operation of the invention is as follows. At the locationof the blade profiles 1 and 2 near the outer blade portions, thethickest nose portion d of blade 2 is disposed opposite the thickestthroat portion 1 of blade 1 and the two together form aconvergent-divergent nozzle of transonic flow conditions indicated at Ibetween the dashed lines. Adjacent zone I is a zone II of supersonicflow and zone III of supersonic flow at essentially constant velocity.The radius of curvature of the concave suction side 12 of blade and theradius of curvature of the concave pressure side 10 of blade 1 are bothcalculated, preferably using the method of characteristics, which iswell know to those skilled in the art, to provide optimum flowconditions.

One advantage of the foregoing design, in having a nose section and atail section which are substantially the same thickness, is to provide amore symmetrical bucket of greater strength and to allow enough materialfor shielding against erosion on leading edge at the outer tips. Also,by employing flat profiles over approximately half of the pressure sideand approximately 4 of the suction side, simplifications as to bladeshaping are introduced.

Well controlled transition from a transonic flow to a supersonic flow ina compressed region between overlapped buckets, such that the staticpressures along suction and pressure side of section 5 are converginggradually to the downstream static pressure, adds greatly to the overallefficiency of the transonic bucket described in relation to previouslyknown designs.

While there is shown what is considered at present to be the preferredembodiment of the invention, it is of course understood that variousother modifications may be made therein, and it is intended to cover inthe appended claims all such modifications as fall within the truespirit and scope of the invention.

What We claim as new and desire to secure by Letters Patent of theUnited States is:

1. In an axial flow turbine having buckets with tips thereon travelingwith respect to elastic fluid passing therethrough under transonicconditions, each of said bucket outer portions being shaped to define ablade profile having a rounded nose section, a mid-section and a tailsection extending over approximately the first quarter, the middle halfand the last quarter of the chord length respectively,

said nose section increasing and then decreasing in thickness to providea first reference point of maximum thickness therein,

said mid-section having substantially fiat sides increasing in thicknessthroughout its length to provide a second reference point of maximumthickness between the mid-section and tail section,

said first and second reference points being of approximately the sameblade thickness.

2. In an axial flow turbine having buckets with tips thereon travelingwith respect to elastic fluid passing therethrough under transonic andsupersonic flow conditions, each of said bucket outer portions beingshaped to define a blade profile having a rounded nose section extendingover approximately one-quarter of the chord length, a mid-sectionextending over approximately onehalf of the chord length, and a tailsection extending the remaining one-quarter of the chord length,

said nose section disposed in overlapping and coextensive relationshipwith the tail section of an adjacent blade profile,

said nose section gradually increasing in thickness commencing at theleading edge thereof and then decreasing in thickness,

said mid-section defining substantially flat surfaces on the pressureand suction sides of the profile and also diverging slightly toward thetail section,

said tail section having a substantially fiat suction side which is acontinuation of said mid-section suction side and having a surface onthe pressure side which gently converges from a maximum throat thicknessto the bucket trailing edge.

3. The combination according to claim 2, wherein the contour of the nosesection from the leading edge to the thickest nose portion isquasi-elliptical and substantially symmetrical on the pressure andsuction sides.

4. The combination according to claim 2, where the maximum thickness ofthe blade in the nose section and the maximum thickness of the blade atthe commencement of the tail section are substantially the same.

5. The combination according to claim 2, wherein the surface of thesuction side of the nose section of one blade and the surface of thepressure side of the tail section of an adjacent blade are disposedopposite one another and are slightly concave and shaped to obtain aconfined supersonic flow region between blades.

References Cited

